U.S. Pat. No. 6,465,557 B1 (KRATON Polymers U.S. LLC) discloses a hot-melt, pressure sensitive positioning adhesive for use in an adsorbent article such as sanitary napkins, incontinent pads, feminine pads, panty shields and diaper inserts.
Said adhesive comprises:
a. from 6 to 15 percent by weight of a hydrogenated styrene-(butadiene and/or isoprene)-styrene block copolymer, having a vinyl content of greater than 50% by weight,
b. from 50 to 80 percent by weight of a tackifying resin, which has an aromaticity such that the MMAP cloud point is at least 45° C., and
c. from 5 to 35 percent by weight of a plasticizer,
wherein the weight percentages of the total components a), b) and c) and the sum of the three components add up to 100.
In U.S. Pat. No. 6,465,557 B1, the preferred vinyl contents in the block copolymer before hydrogenation are those in the range of from 70 to 80 wt %, while the preferred block copolymer content in the adhesive composition is from 8 to 11 percent by weight. Preferred poly(styrene) contents in the block copolymer are in the range of from 10 to 40 percent by weight.
A clear disadvantage of the described and exemplified adhesive compositions of U.S. Pat. No. 6,465,557 B1 is that they comprise large amounts of plasticizing oils such as TUFFLO 6056, which have been found to leave residues on the surface to be protected by the protective film after the film is removed. It will be appreciated that these residues on the surfaces to be protected are unacceptable for several types of products.
U.S. Pat. No. 5,427,850 (Sekisui Chemical Co.) discloses a pressure sensitive adhesive composition comprising 100 parts by weight of at least one block copolymer selected from the group consisting of block copolymers represented by the general formula A-B-A and block copolymers represented by the general formula A-B, wherein A denotes a styrene polymer block and B denotes a butadiene polymer block, an isoprene polymer block or a polymer block obtained by hydrogenating these polymers, 10 to 200 parts by weight of a tackifying resin and 25 to 200 parts by weight of a polyolefin. Pressure sensitive adhesive tapes or sheets comprising, as a pressure sensitive adhesive layer, the pressure sensitive adhesive composition are also disclosed.
WO 00/77118 (KRATON Polymers Research BV) discloses an adhesive composition comprising: (i) a block copolymer containing at least two poly(monovinyl aromatic hydrocarbon) blocks and at least one hydrogenated poly(conjugated diene) block; (ii) 20 to 80 parts by weight of an at least partially hydrogenated tackifying resin per 100 parts by weight of block copolymer; (iii) 0 to 40 parts by weight of aromatic resin per 100 parts by weight of block copolymer; (iv) 10 to 60 parts by weight of a polybutene-1 per 100 parts by weight of block copolymer; and (v) 0 to 25 parts by weight of plasticizer per 100 parts by weight of block copolymer. Said document further discloses a process for preparing pellets containing the adhesive composition and a protective film comprising a substrate layer and an adhesive layer comprising the adhesive composition as defined above, made for instance, by co-extrusion.
The compositions according to U.S. Pat. No. 5,427,850 and WO 00/77118 have in common that they comprise a styrenic block copolymer, a midblock compatible tackifying resin (i.e., compatible with the hydrogenated poly(conjugated diene) block) and a polyolefin. The latter is generally the cheapest component of the composition. It is therefore used in large amounts. On the other hand, the polyolefin does not contribute to the desirable properties of the adhesive composition. For instance, producers of protective films are on the look out for adhesive compositions that may contain a lot of cheap polyolefins while still having excellent adhesive properties (generally determined on the basis of peal adhesion) and good processability (generally determined on the basis of Melt Flow Rate).
Accordingly, it would be desirable to have a pressure sensitive adhesive composition that combines a high peel adhesion with a Melt Flow Rate (MFR according to ASTM D-1238, Condition E, 190° C. at 2.16 kg load) from 5 to 20 g/10 min.